The present invention relates to a process and a device for induction welding parts made of composite materials. The invention relates in particular to the assembly of parts made of composite materials in the aeronautical field.
The parts made of composite materials generally comprise carbon fibers embedded in a resin which are deposited by drape forming, for example, so as to create folds superposed one on top of the other. In each fold the fibers are aligned in a main direction (0°, 45°, 90°). The planes of the carbon fiber folds are parallel to one another and define a plane referred to as the principal plane. Moreover, the methods involved in producing a part of this kind generate points of contact between the folds, the distribution of said points of contact being random and depending on the nature of the resin and the speed at which the carbon fibers are deposited.
The induction welding of parts made of thermoplastic composite materials involves using an inductor that emits a magnetic field and an interface element, usually made of conductive metal, disposed at the interface of the parts to be welded to focus the magnetic fields. The conductive metal is heated to what is referred to as a process temperature which at least corresponds to the transformation temperature of the composite materials, causing the structure of the composite parts to soften, in order to mix the two parts locally and thereby allow them to be welded together. The parts to be welded together must be made up of “compatible” composite materials, in other words composite materials that adhere to one another at the transformation temperature.
However, the main disadvantages of this type of process are firstly the highly localized presence of conductive metal in the final product and secondly, the difficulty in controlling the temperature of the process.
Certain induction welding devices cause the currents induced in the carbon of the surface folds of the composite materials to circulate. These devices generally tend to heat the contact walls, causing currents induced in the paramagnetic materials to circulate, such as the carbon of the carbon fibers of the composite materials, in other words with low sensitivity to the magnetic fields and with an electrical conductivity that allows the circulation of the Foucault currents to be transformed in terms of temperature by the Joule effect. This involves applying strong magnetic fields, in order to generate induced currents in these materials. Induced currents can likewise circulate in the folds which are not at the interface, as well as along points of contact between the folds.
In parts made of composite materials that are subject to the magnetic field of an induction welding device, the induced currents circulate in two directions:                the main direction in the folds, along the carbon fibers of the parts, and        a secondary direction perpendicular to the main direction between the folds, along which the points of contact are positioned between folds.        
These welding processes through the circulation of induced currents are therefore difficult to control in terms of temperature and welding interface. In fact, it is difficult to manage the magnetic field in order to limit its effect exclusively to the surface carbon folds which are at the interface between the two composite materials to be welded.
Patent EP1326741B1 describes a polymer matrix composition comprising ferromagnetic elements dispersed in a polymer matrix and a process allowing heat to be generated by uniform hysteresis in the composition. With a matrix of this kind, the heating of the polymer matrix is uniform, localized and precisely controlled in terms of temperature.
Moreover, the induction welding of thermoplastic composites, in particular high-performance composites, is performed in the usual manner with the help of an inductor exhibiting a single-sided configuration.
Processes of this kind, with or without conductive metal, are prone to several limitations which prevent their use on an industrial scale in the aeronautical field.
In effect, the geometric design of the inductor is complex for simple planar surface geometries. It requires simulations that call for substantial computer resources, for example an L-shape requires two weeks of simulation in order to obtain the shape of the inductor, without guaranteeing the uniformity of the heat effect.
Moreover, there is a strong sensitivity to the distance between the inductor and the interface element which influences the heat balance at the interface. The magnetic fields emitted by the inductor are initially very strong in its immediate environment, decreasing exponentially as they move away from the inductor. This configuration involves geometric sensitivity, in other words, a tolerance in the distance between the inductor and the metal of less than one-tenth of a millimeter. In the case of a one-sided inductor, this distance has a great effect on the heat balance at the interface. If this distance varies, the temperature is too great or, conversely, is not great enough.
Hence, there is a close link between the geometry of the one-sided inductor and those of the parts to be welded. A different inductor has to be designed for each configuration with an adaptation of the welding parameters, such as the frequency, field strength or inductive system impedance.